JSON API, OpenAPI and JSON Schema Working in Harmony

Phil Sturgeon

Sep 24 2018 · 4 min read

A regular question on Twitter, at WeWork, and in the APIs You Won’t Hate Slack
community, is: “What standard should my API
follow? Should it be JSON API or something else?” At which point a bunch of
people usually start discussing things like OpenAPI and/or JSON Schema. In this
article, I’d like to talk about how they can all work together in harmony, to do
different things.

JSON API

JSON API is a specification written by a group of folks,
with the goal of being an anti-bikeshedding tool for
writing JSON APIs. The name often causes a bit of confusion, but JSON API is one
of many data formats that is often applied to REST (or RESTish) APIs, as an
alternative to Siren, HAL, Uber,
etc.

The goal of JSON API is to standardize some of the specifics of API design that
the REST paradigm leaves to the implementer. REST has no opinions on how you
implement resources vs collections, or where meta data should go, how to include
related resources, how pagination should work, or anything else, so JSON API
tries to fill in a lot of those gaps for you.

Many people try to figure those things out their selves, and there are a lot of
things that can go wrong. Build APIs You Won’t
Hate would
have been a lot slimmer if everyone had just used JSON API back then.

Firstly, JSON API explains what shape the body of the HTTP request and response
should take. Specifically, where primary data goes, where meta data goes, where
should links to other resources be placed, and how exactly should related data
be included.

This structure initially seems like a lot of noise to somebody who is expecting
an API to do one thing: transfer a few fields from the server to their client,
but when an API is more of astate machine over
HTTP,
a lot of this starts to make sense.

Secondly, beyond just the shape of the request/response body, JSON API helps
with a few other things, like Sparse
Fieldsets. You can let
clients pass /articles?fields[articles]=title,body to get just the title and
body fields, a feature much loved by GraphQL advocates. They’re a controversial
topic in the REST world because they can slim down responses, but also screw up
cache
ratios.
Maybe use them maybe don’t, but JSON API is there to give you the option for
this. It helps out with a lot of other things, like ?includes= for compound
documents, it takes a
stab at pagination, makes a
vague hand-wave at filtering,
and covers things like error
objects for those not using
RFC 7807.

All of this is basically very structural, but none of it tells you anything
about what the data is, or anything about the data. There is no “schema”
functionality (or types, as some folks call them). For that, you need to look at
something like OpenAPI, or JSON Schema.

OpenAPI and JSON Schema

OpenAPI and JSON Schema are
two different specifications that have inspired each other a lot, but are subtly
different.

OpenAPI aims to describe both the service model (the API in general, endpoints,
request metadata like headers, authentication strategies, response metadata,
etc., and it also covers the HTTP request/response body using a bunch of
keywords based on JSON Schema, that have diverged over
time.
The divergence is slowly being
solved.

JSON Schema aims to describe an instance of JSON data, like the ones found in a
HTTP request or response, but is in no way limited to a HTTP API.

In describing the data, you can say which fields are required, mention common
formats like email, UUID, etc., add more complex validation rules to those
fields like maximum length, or regex patterns, and all sorts of other
functionality. The data could be in any shape, but whatever data is there can be
described with one of these two tools.

At WeWork, we use JSON Schema to describe the data models, OpenAPI to describe
everything else, then the message being described is usually JSON API.

OpenAPI has a lot of design-time and build-time
tooling, so it’s popularly used for mocking services
and generating SDKs. It is not commonly used for run-time functionality. For
that, most folks use JSON Schema, which can do many amazing things like
client-side
validation.

Seeing as JSON Schema does not touch the service model, it’s quite tough to
build things like SDK generators as they do not know anything about the
endpoints. There could be a future where JSON HyperSchema can take over this
role from OpenAPI, as a fully RESTful API with JSON HyperSchema would be able to
build a HATEOAS compatible SDK, which would focus on navigating from root, to
resource, to resource, instead of worrying about memorizing endpoints in the
first place.

JSON API and JSON HyperSchema can actually complement each other quite well.
Aaron Hedges recently wrote about how JSON HyperSchema can be used to describe
JSON API
payloads,
taking the plain old “here is a HTTP link do with it what you will” approach to
HATEOAS, and peppering it with all sorts of useful information.

Some folks refer to this as upgrading an APIs “Hypermedia Maturity Model”,
taking JSON API from HMM 1 to HMM 2. More on the Hypermedia Maturity Model
here.

Summary

The point here is that these three things all have rather different jobs, and
can be used together. You don’t want multiple teams cranking out handfuls of
REST APIs with completely different payload shapes and completely different
approaches to things, and whatever shape the API data is in, you need to know
more about what that data actually is. Think of one as a “Data Format” and
another as a “Data Contract” and things make a bunch more sense.

I’ve used JSON API extensively for years, flipping from being a big fan to an
outspoken critic, and back again multiple times. JSON API is a great way to
solve a lot of problems in a HTTP/1 world, butHTTP/2 should handle a lot of
the things that JSON API focuses
on.
That said, a lot of tooling (especially in the Ruby world) is still completely
ignoring HTTP/2. I recommend JSON API as an alternative to anarchy, but it is
full of potential foot-guns, like any powerful concept used carelessly. Please
readMaking the Most of JSON
APIif you are considering using it, and do not take my mentioning of it here as
tacit approval for every idea in the specification.

The cover photo is an awesome
[https://www.etsy.com/listing/121737141/zelda-triforce-lamp](Zelda Triforce
Lamp) which I would absolutely buy if I lived somewhere.